For example, Japanese Patent Kokai Publication No. (1993)312156 discloses, as an example of compressors which decrease the volume of refrigerant in a refrigeration cycle, a scroll type compressor. A typical scroll type compressor includes a compressing mechanism including within its casing a stationary scroll having a projectingly formed spiral wrap and a movable scroll having a projectingly-formed spiral wrap, wherein the wrap of the movable scroll is intermeshed with the wrap of the stationary scroll. The stationary scroll is firmly secured to the casing. On the other hand, the movable scroll is linked to an eccentric shaft part of a drive shaft.
The movable scroll does not rotate on its axis but executes only an orbital motion relative to the stationary scroll. With the orbital motion of the movable scroll, the volume of a compression chamber formed between the wraps decreases, so that the refrigerant in the compression chamber is compressed.
Incidentally, when refrigerant is compressed in such a scroll type compressor, this causes both a thrust load which is an axial force and a radial load which is a lateral force orthogonal to the thrust load to act on the movable scroll. More specifically, the thrust load acts on a thrust bearing located between an end plate of the stationary scroll and an end plate of the movable scroll and, as a result, the movable scroll is forced to be drawn apart from the stationary scroll. In order to resist the thrust load, there are provided a high pressure gas chamber divisionally formed on the end plate rear surface side of the movable scroll and a high pressure fluid operation space (fluid chamber) to which high pressure fluid is supplied from a high pressure fluid supplying means. A back pressure of the pressure of a high pressure fluid in the fluid chamber and the pressure of a high pressure gas acts as a pressing force that presses the movable scroll in the direction of the stationary scroll. Here, in some cases such a pressing force is small and the vector of a resultant force acting on the movable scroll may pass outside the outer peripheral surface of the thrust bearing. This gives rise to the problem that the movable scroll becomes inclined (overturned) by the action of a so-called upsetting moment and, as a result, there occurs a refrigerant leak, thereby causing a drop in efficiency.
In order to deal with such a problem, an increased back pressure more than a predetermined level is impressed on the movable scroll. Pressing force by the back pressure is determined by the dimensional constraint of a seal ring and the setting of overturn limitation, and however in some cases there may occur an excessive pressing force during the high speed operation. In order to cope with this problem, there has been proposed a construction in which high pressure fluid is introduced to the thrust bearing between the stationary scroll and the movable scroll, with a view to reducing the pressing force.